This invention relates to an image pick-up tube target and, more particularly to an improved structure of a rectifying contact type image pick-up tube target comprising a P-type photoconductive layer containing Se as a major constituent, and also As and Te or other materials and a beam landing layer, featuring little variation in signal current-target voltage characteristic during a long term operation.
A photoconductive image pick-up tube target utilizing a rectifying contact between a P-type photoconductive layer comprising amorphous Se, Te, As and other materials and N-type material is disclosed in U.S. Pat. Nos. 3,890,525, 3,922,579, 3,984,722, 4,040,985, and 4,330,733.
The advantages of an image pick-up tube target of this type are high speed response, little image flare, high resolution and ease of manufacture.
FIG. 1 shows a section of a main part of an image pick-up tube target. A transparent conductive film 2 containing SnO.sub.2 or In.sub.2 O.sub.3 as a major constituent is formed on the rear surface of a transparent glass substrate 1. A very thin N-type transparent conductive film 3 of zinc selenide, germanium oxide, cerium oxide, lithium fluoride or the like is formed on the rear surface of the transparent conductive film 2. A P-type photoconductive film 14 of several microns in thickness containing amorphous Se as a major constituent is deposited on the N-type transparent conductive film 3 to form a rectifying contact with the N-type conductive film 3. A beam landing layer 5 of Sb.sub.2 S.sub.3 or the like is deposited to be about 1000 .ANG. thick on the P-type photoconductive layer 14.
Incident light 6 from a scene enters the transparent glass substrate 1 and a scanning beam 7 lands on the Sb.sub.2 S.sub.3 layer 5 to convert the light image into electrical signals.
Since Se is not sensitive to long wavelength radiation, it has been proposed to add Te into a portion of several hundreds .ANG. in thickness near the rectifying contact of the Se-containing layer 14 with a peak concentration of 20 to 50% by weight in order to improve the sensitivity to long wavelength radiation. This method is disclosed in U.S. Pat. Nos. 3,890,525 and 4,040,985.
The amorphous photoconductive layer 14 with Se as a main component is easily crystallized by heat, resulting in appearance of white dots in a reproduced picture. As is doped throughout the P-type photoconductive layer 14 in order to increase the viscosity and to decrease the speed of crystallization. In this case, the P-type photoconductive layer 14 is formed by evaporation of a compound containing a predetermined amount of Se and As, or by alternate evaporation of thin layers less than 100 .ANG. of Se and As.sub.2 Se.sub.3 as disclosed in U.S. Pat. No. 3,800,194.
An image pick-up tube target of the aforementioned conventional type leaves much to be desired in terms of sensitivity- and sensitivity saturation-target voltage characteristics and suppression of dark current. A poor sensitivity saturation characteristic requires a higher target voltage to be applied, and increase in dark current results in degradation of a reproduced picture.
U.S. Pat. No. 4,219,831 discloses a method to improve sensitivity- and sensitivity saturation-target voltage characteristics and to suppress dark current by limiting a whole amount of As doped into the P-type photoconductive layer in a range between 2.5 and 6% by weight.
Japanese Patent Publication (Kokoku) No. SHO. 57-44030 discloses a method to prevent cracking of a beam landing layer made of Sb.sub.2 S.sub.3 due to rapid temperature changes or operation under high temperature conditions by doping As of 10 to 20% by weight in the region of the P-type photoconductive layer of about the same thickness as that of the beam landing layer of Sb.sub.2 S.sub.3 and adjacent to the beam landing layer.
However, the aforementioned conventional image pick-up tube targets had signal current-target voltage characteristics varied during a long term continuous operation due to changes in internal electric field within the target, resulting in deterioration of picture quality.
It has been one of the demands by TV camera users to improve the signal current-target voltage characteristic for the purpose of reproduction of higher quality pictures.